1. Field of the Invention
The formation of compositions of sulfonate containing polymers have been clearly defined in a number of U.S. Pat. Nos.: 3,642,728; 3,836,511; 3,847,854; 3,870,841; and 3,877,530. The compositions of these patents are formed from sulfonate polymers which are formed by contacting a polymer having olefinic unsaturation with a sulfonating agent.
The compositions of this patent differ from the previously identified compositions in the aforementioned patents in that the polymers of the instant compositions are formed by free radical emulsion polymerization of at least one conjugated diene with a sulfonate containing monomer to form a substantially gel-free co- or terpolymer having a major portion of a conjugated diene and a minor portion of a sulfonate containing monomer, wherein the co- or terpolymer is water insoluble.
2. Prior Art
Copolymers and terpolymers containing sulfonate monomers at low levels have been described previously in the art. For example, U.S. Pat. No. 3,306,871 describes the preparation of latices based on several different vinyl monomers. This prior art is particularly relevant to the instant invention because it distinguishes those features which are characteristic of much of the prior art in the area of metal sulfonate ionomers.
It has become clear based on recent work that sulfonate ionomers can manifest many peculiarities which make their characterization extremely difficult. The foremost of these is a strong ionic crosslinking which makes such materials difficult to dissolve in solution or to achieve melt flow.
This particular problem is exacerbated even more when describing copolymers of diene monomers and metal or amine sulfonate containing monomers for in this latter case the probability of a high degree of covalent crosslinking is extremely likely. When concurrent problems of ionic crosslinking and covalent crosslinking are now combined with problems of uniform copolymers obtained between a relatively nonpolar hydrocarbon monomer and a highly polar salt molecule which are normally completely immiscible, and the normal problems of molecular weight control desired of all polymer systems, it becomes evident why there is a paucity of information concerning the characteristics of the isolated sulfonate ionomers obtained by copolymerization of dienes and metal or amine sulfonate containing monomers. In fact, a review of the patent art and the literature suggests that a complete characterization of such systems has not often, if ever, been previously attempted.
Possibly for the above reasons most of the patent art in this area describes the latexes achieved by the presumed reaction of such sulfonate monomers with selected vinyl and diene monomers. That the products are copolymers is presumed by suggestions that the resulting latexes are more stable, more water resistant, and more adherent than those obtained in the absence of the sulfonate monomer.
The instant invention is directed at compositions of a different class of copolymers of dienes and metal or amine sulfonate containing monomers. This invention is concerned with the preparation and resultant compositions of co- or terpolymers of dienes and sulfonate monomers which are of desirable molecular weights, are substantially free of covalent crosslinking (less than 10% of the product appears as a gel in prescribed tests), contain sulfonate levels of from 0.5 weight percent chemically combined sulfur up to 5 weight percent chemically combined sulfur, are water insoluble, and are prepared by a process designed to give products substantially free of homopolymers of any of the polymerizable components, and are solid products, characterizable in terms of reduced viscosity, molecular weight and/or melt flow (at elevated temperatures).
In light of the above distinguishing features it is readily apparent as to why the products and process of the instant invention differ from those of GB Pat. No. 895,033. That patent addresses specifically latexes based on copolymerization of a suitable aromatic vinyl sulfonic acid derivative with a variety of different polymerizable monomers. It is important to note that all aspects of that invention are solely concerned with the resultant latexes, that the products are isolated entities.
For the purposes of the instant invention many of the features of the products in GB No. 895,033 are undesirable. For example, it is emphasized in that application that improved latex stability is an asset, for the purpose of the instant invention that improved latex stability can be a debit in that difficulties may be encountered in isolating the solid polymer.
Another feature of the instant invention is that the instant disclosure is directed towards products which are demonstrated to be substantially free of covalent crosslinking and techniques whereby the ionic crosslinking desired in such systems can be separated from the covalent crosslinking. This demonstration has not been illustrated in the prior art to copolymers of sulfonate containing monomers with polymerizable conjugated diolefins. Without this illustration it would be difficult if not impossible to employ the resultant products in some of their intended applications.
Finally, and most importantly, U.S. Pat. No. 3,306,871 and GB No. 895,033 specifically state in column 1, second paragraph of both patents that those inventions were concerned with polymer latexes wherein the sulfonic acid salt is incorporated as an anionic stabilizer. It is important to emphasize that in the instant invention the sulfonate is incorporated at specific levels to function as an ionic crosslinking agent. By that we mean that in these systems the neutralized sulfonate provides a salt species which interacts with other salt species to provide a strong ionic crosslink. This characteristic is observed and desired, not in the latex form, but is extremely important in determining the bulk physical properties, especially in the compositions to be described herein.
In light of the above discussions it is apparent that the products of this invention differ markedly from those of the prior art in properties, in composition and in their specific structural features.
In addition, the catalysts or initiators employed, for example. in GB No. 895,033 differ from those employed in the process of the instant invention. The prior art has suggested that either water soluble free radical-generating catalysts or oil soluble free-radical generating catalysts are suitable. Alternatively, U.S. Pat. No. 2,913,429 specifies that "it is necessary that the polymerization mixture include a water soluble peroxy compound as a catalyst and it is desirable, but not essential, that it also include an oil soluble catalyst ingredient" (column 2, line 28 forward).
The initiators of the instant invention are important and different from those of the prior art for it is believed that they function to give an optimum copolymerization without significant incorporation of either a homopolymer of the hydrocarbon molecule or a homopolymer of the metal sulfonate monomer. If one employs substantial levels of an oil-soluble initiator which can spontaneously polymerize solely in the diene phase then that species can be polymerized without a corresponding incorporation of the sulfonate species. By appropriate selection of one catalyst component soluble in the polymerizable hydrocarbon phase and a second catalyst component (reducing agent) in the aqueous phase then the interaction of these components can result in a more uniform copolymerization predominantly at the interface.
Another patent which can be considered to be relevant to the instant invention and which teaches ionic crosslinking is that of Rees, U.S. Pat. No. 3,322,734. U.S. Pat. No. 3,322,734 teaches that ionically crosslinked copolymers may be prepared via direct copolymerization and teaches how such ionic crosslinking can change the properties of polymers. However, that patent specifically is directed at neutralization levels of between 10% and 90% of the acid species present. The instant invention is directed at neutralization levels of 95% and above, and preferably at ionomers which are 100% neutralized. The properties of the resulting material which are 100% neutralized are substantially different from those which are only 50% or 90% neutralized. Therefore, the instant invention is substantially different from that of U.S. Pat. No. 3,322,734.
In addition to the above art, mention should be made of U.S. Pat. No. 2,913,429 which is concerned with synthetic latexes designed to form films which are based on aqueous dispersions of copolymers of one or more aliphatic conjugated diolefins with at least two monovinyl aromatic compounds including a monovinyl aromatic sulfonate etc. This invention differs from that prior art in the following:
(1) This application is not concerned with films from latexes. PA1 (2) That invention contains from 4 to 35, preferably from 5 to 15 weight percent sulfur monomer and thereby provides coatings or films which can readily be removed from substrates by washing or scrubbing with water. Obviously, these systems are designed to be water sensitive and thereby removable, the compositions of the instant invention are designed to be water insensitive. PA1 (3) That cited patent required 93 to 25, preferably from 77 to 45% vinyl aromatic compound such as styrene. The instant application does not permit more than 48% styrene, preferably from about 20 to about 40 weight percent. PA1 (4) That invention requires a water soluble peroxy compound as initiator whereas this application required a hydrocarbon soluble peroxy initiator and a water soluble reducing agent.
There are many other distinctions but it is obvious that the above invention is directed at essentially rigid removable paint films, wherein the sulfonate groups provide adequate water sensitivity to permit the formation of a stable latex which further can be deposited as a removable film. Nowhere in that invention is the concept of a metal sulfonate copolymer possessing strong ionic crosslinks in the bulk product taught, inferred, or even desired.
The use of metal stearate and similar derivatives as plasticizers for metal sulfonated polymers has been described in previous and issued U.S. Pat. Nos. 3,642,728 and 3,870,841. Consequences of plasticization of such polymers with metal stearates are improved flow at elevated temperatures and improved tensile properties at ambient temperature. While the mechanism for these improvements is not completely clear, it is apparent that major improvements of the physical property-rheology balance of metal sulfonate elastomers and plastics are feasible by this approach.
It should be emphasized that the prior art relating to these plasticization concepts has been based on the sulfonation of preformed polymers, followed by neutralization. The instant invention is concerned with the plasticization of copolymers of sulfonate monomers with selected diene monomer or terpolymers which may involve a third monomer such as styrene. It is important to recognize some major distinctions of these materials as compared to those of the prior art. The most important one is that prior to this invention there is no evidence that plasticization of sulfonate copolymers was ever attempted. Secondly, the plasticization of these polymers appears markedly different in concept and in practice from the plasticization attempts of other types of sulfonated polymers. The sulfonate copolymers employed in this invention have modest sulfonate levels of about 5 to about 300 meq. per 100 gms of polymer with the major portion of the polymer molecule being based on the diene. Consequently, these products have a very high level of unsaturation, and therefore, have different requirements for plasticization than do sulfonated polymers. For example, when exposed to processing temperatures of 300.degree. to 400.degree. F., the polymers of the instant invention can crosslink extremely readily. This characteristic thus sets such systems apart from the sulfonated ethylene propylene terpolymers (EPDM) previously described.
The molecular weights and molecular weight distribution of the sulfonate copolymers are also quite different from those characteristics of the sulfonate polymers. This difference has an important effect on the properties and flow behavior of the products. In addition, properties such as low temperature glass transition or brittleness temperature are different for the products of this invention from those previously described.
Finally, due to the way in which these products are created the sulfonate level can be much higher (or even lower) than those products obtained by sulfonation of a preformed polymer. For example, the sulfonation of EPDM requires a certain level of unsaturation to effect the necessary sulfonate level. Practically this provides an upper limit of sulfonation to about 40 to 60 meq. of sulfonate per 100 gms of polymer. The sulfonate copolymers employed in this invention can be substantially higher in sulfonate content which makes the plasticization requirements more critical.